A comparative study on the effect of chilling treatment in the light and in the dark on subsequent photosynthesis in cucumber

The adverse effect of chilling treatment (4 degreesC) in the light and dark on the subsequent photosynthesis was compared by measuring O-2 evolution a nd modulated chlorophyll (Chl) fluorescence using leaf discs taken from chi lled cucumber (Cucumis sativus L.) plants. Weak light (50 mu mol m(-2) s(-1 )) was adopted for light chilling to discount the photoinhibitory effect, w hich was substantiated by no decline in the ratio variable/maximal fluoresc ence after dark adaptation (F-v/F-m) after light chilling. The inhibitory p ace was faster in the light. Decrease in photosynthesis was not prominent f or the initial 2 h of chilling in the light or 12 h of chilling in the dark , but was manifested as an abrupt drop thereafter. Approximately 50 and 90% inhibition of O-2 evolution was observed after chilling for 4 and 6 h, res pectively, in the light, but the same degree of inhibition was shown only a fter 24 and 48 h of chilling in the dark. Chl fluorescence measurement show ed no significant change in initial fluorescence (F-o) and F-v/F-m, but not able change in quenching parameters. Chloroplasts isolated from either 4-h light-chilled or 24-h dark-chilled plants showed roughly 50% reduction in C O2 fixation. Results with reconstituted chloroplasts revealed damage in thy lakoids from light-chilled plants. In contrast, stroma from dark-chilled pl ants was less functional. Electron transport was hampered only in light-chi lled plants with most lesions residing in photosystem I (PSI). Separation o f photosynthates showed changes in metabolite distribution, but more substa ntially in the dark-chilled chloroplasts, indicating that stromal enzymes w ere affected. These results suggest that chilling-stress in the light prima rily interferes with thylakoidal function while that in the dark mostly aff ects stromal function within the chloroplasts.